An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
Different types of bits work more efficiently against different formation hardnesses. For example, bits containing inserts that are designed to shear the formation frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDC's) as their cutting faces.
Roller cone bits are efficient and effective for drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in that the inserts of the rotating cones are impacted against the formation material as the cones rotate. This action loads the formation material beyond its compressive strength and allows the bit to cut through the formation.
For still harder materials, the mechanism for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations such as sandstone. For these hard formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are effective. In the discussion that follows, components of this type are referred to as “diamond impregnated.”
During abrasive drilling with a diamond-impregnated cutting structure, the diamond particles scour or abrade away concentric grooves while the rock formation adjacent the grooves is fractured and removed. As the matrix material that supports the diamond granules is worn away, the diamonds at the surface eventually fall out and other diamond particles are exposed.
To form a diamond-impregnated bit, diamonds, which are available in a wide variety of shapes and grades, are placed in predefined locations in a bit mold. Alternatively, composite components, or segments comprising diamond particles in a matrix material such as tungsten carbide/cobalt (WC—Co) can be placed in predefined locations in the mold. Once the diamond-containing components have been positioned in the mold, other components of the bit are positioned in the mold. Specifically, the steel shank of the bit is supported in its proper position in the mold cavity along with any other necessary formers, e.g. those used to form holes to receive fluid nozzles. The remainder of the cavity is filled with a charge of tungsten carbide powder. Finally, a binder, and more specifically an infiltrant, typically a nickel brass alloy, is placed on top of the charge of powder. The mold is then heated sufficiently to melt the infiltrant and held at an elevated temperature for a sufficient period to allow it to flow into and bind the powder matrix or matrix and segments. By this process, a monolithic bit body that incorporates the desired components is formed.
In conventional diamond impregnated bits, as described above, the bits include diamond impregnated ribs that support diamond-impregnated inserts. It has been found that, as the inserts in these bits wear down to the level at which they are embedded in the bit body, the diamond-impregnated structure surrounding each insert begins to contact the formation. Because the support structure is diamond-impregnated, it is wear-resistant and causes a significant increase in the friction when it begins to contact the formation. Hence, the diamond impregnated ribs present a relatively large contact area as wear progresses. Certain rocks such as carbonates are very hard but relatively non-abrasive when compared to silts and sands. The increased contact area makes drilling such formations ineffective and compromises drilling rates, increasing drilling costs significantly. Hence, it is desired to provide apparatus and methods that mitigate the increased contact area caused by conventional ribs and allow efficient drilling in carbonate and other hard formations.